1. Field of the Invention
The present invention relates generally to an ignition system which includes a knock sensor or the like and which controls the ignition timing in response to the output of the sensor so as to optimize engine operation, and more specifically to such an ignition system which can switch between a control schedule suited for fuels having a normal octane rating (e.g. regular) and a schedule suited for high octane fuel.
2. Description of the Prior Art
In order to meet the ever tightening air pollution requirements the use of lead additives (e.g. tetraethyl lead) has been restricted thus inducing a tendency toward the use of low octane lead-free fuel. However, engines designed for operation on such fuels inevitably have low compression ratios so as to avoid knocking. However, as is well known, low compression engines lack desired performance characteristics and thus a demand for an engine which can be operated on lead free high octane fuel has arisen.
However, a drawback with the just mentioned type of engine occurs in that, if for any reason the engine should be supplied with low octane fuel, severe knocking tends to occur and sometimes leads to engine damage.
In order to overcome this problem it has been proposed to provide the engine with an ingition system which can be switched between an ignition control schedule suited for high octane fuel to one which is suited for low octane fuel (note that low octane fuel will be referred to as regular hereinafter). However, this switching is effected manually, thus inducing the possibility of human error.
Accordingly, in Japanese Patent Application First Provisional Publication No. 58-143169 an arrangement has been proposed wherein the ignition schedule suited for high octane fuel is initially used and, if under a predetermined mode of engine operation, knocking is detected (indicating the use of regular fuel), the system is switched over to a schedule suited for low octane or regular fuel. Viz., if knocking is detected under a predetermined mode of engine operation the system senses the use of regular fuel and switches the ignition control schedule. This newly selected schedule is subsequently maintained either for a predetermined period of time or until the engine is stopped.
However, with this system it is possible that the engine will directly enter a high power mode of operation and thus not pass through the mode in which the decision to switch from the high octane control schedule to the one suited for regular fuel is made. Under such instances, due to the highly advanced ignition timing used with high octane fuel, the combustion of regular fuel tends to produce violent engine damaging knocking.
Further, with the above described system, the degree to which the ignition timing can be advanced or delayed it not limited, whereby, unless the engine knocking is accurately sensed, highly undesirable knocking can occur due to excessive ignition timing advance. To avoid this problem it is necessary to accurately distinguish knocking from background noise. Thus the need arises for rather complex and expensive sensor apparatus.